In mass spectrometry, it is common to have analyte ions of interest obscured or interfered with by ions which have the same nominal mass-to-charge (m/z) value, or by ions that have an m/z value which cannot be resolved from that of the analyte ion by the mass spectrometer being used. These are called isobaric or spectral interferences. Such interferences are common in many types of mass spectrometers, including for example those which use a plasma ion source, those which use glow discharge ion sources, and those which use electrospray and ion spray sources.
Isobaric or spectral background interferences may typically arise from the plasma itself, typical interfering ions being Ar.sup.+, ArO.sup.+, Ar.sub.2.sup.+, ArCl.sup.+, ArH.sup.+, ClO.sup.+, and MAr.sup.+ (where M is a sample matrix element, i.e. a dominant species of ion in the collection of ions), MO.sup.+, and the like. Such interfering ions may also form in the extraction process (possibly due in part to the cooling of the plasma during expansion into vacuum, or due to interaction with the sampler or skimmer orifices), or within the momentum boundaries which exist at the edges of the sampler or skimmer.
Fragmentation of polyatomic ions in the collision cell can give rise to further or enhanced isobaric (spectral) interferences. Reaction of plasma ions with the collision gas used in a multipole device or collision cell may also give rise to spectral background interference, as can ionization of contaminant species derived from the collision cell or vacuum chamber or from contaminants in the collision gas.
It has commonly been thought that one solution to the isobaric interference problem is to use a high mass resolution mass analyzer, but this approach has not been particularly successful and is also hampered by the accompanying loss of ion signal which is inherent in the high resolution approach.
In mass analysis, non-spectral interferences are also commonly encountered. These typically derive from neutral metastable species, and produce an elevated continuum background, i.e. a background which is elevated over a range of masses (so that it is non-spectral). This background adversely affects the detection limit of the instrument.
Therefore, it is an object of the invention in one aspect to provide a method of reducing isobaric and non-spectral interferences in an efficient manner, with reduced ion signal loss, and if desired in addition with relatively high resolution.